An anti-aliasing filter (AAF) is a filter used to restrict the bandwidth of an analog input signal before sampling the analog input signal in order to approximately or completely satisfy the sampling theorem over the frequency band of interest. In other words, the highest frequency of the analog input signal may be limited by the AAF to equal to or below half of the sampling frequency. An AAF is therefore effectively a lowpass filter which limits the highest frequency of the analog input signal.
FIG. 1 illustrates an example of an AAF 100 providing a filtered signal for input into a double sampled ADC 102. It will be appreciated that the AAF 100 may be used to provide an input signal for any suitable type of analog processing circuitry. The AAF in this example comprises a rough low pass filter 103 and a fine low pass filter 104 which provide the two input signals for the rough and fine charging phases of the double sampled ADC 102.
However, ADC anti-aliasing requirements have become fairly stringent recently. This requirement may be challenging for ADCs that are directly connected to the AAFs without any programmable gain amplifiers (PGAs) or any active low pass filter.
In order to improve the anti-aliasing, the capacitance or the resistance in the AAF may be increased. However, increasing the resistance or impedance of the signal path leads to an impact on the total harmonic distortion and increasing the capacitance comes at a significant area cost.
Alternatively, an active low pass filter comprising a programmable gain amplifier (PGA) in the signal path may improve the anti-aliasing. However, the active LPF would also have to meet THD requirements associated with the signal path. Area and power concerns for such an active LPF may be of particular importance as the active LPF would be in the signal path.